1. Field of the Invention
The present invention relates to a switching power supply circuit and a control method used therein. The present invention relates to a switching power supply circuit suitable for an electronic device or the like, for example, driven by a power supply voltage obtained by boosting a relatively low voltage such as that of a battery, and to a control method used in the power supply circuit.
2. Description of the Related Art
In recent years, attendant upon decreases in size of electronic devices, power supply units to be included in the respective electronic devices are also required to be decreased in size. In many cases, a switching power supply circuit is used as such a power supply unit. In general, decreasing the size of such a switching power supply circuit is coped with by raising its switching frequency. However, raising the switching frequency brings about an increase in the switching loss in a switching element and therefore an increase in the heat generation quantity. Thus it is required to increase the size of a radiator or the like to be combined with the switching element. This is an obstacle to decreasing in size. For this reason, it is requested to reduce the switching loss in such a switching element by another method than switching frequency.
Switching power supply circuits are classified into buck type, boost type, and buck-boost type. Of them, a switching power supply circuit of boost type includes a DC power supply unit such as a battery, a choke coil, a switching element a rectifier diode and a smoothing capacitor. In the boost type switching power supply circuit, electromagnetic energy supplied from the DC power supply unit is stored in the choke coil when the switching element is in ON state. Subsequently, when the switching element is turned into OFF state, counter electromotive force generated in the choke coil is superimposed on the output voltage of the DC power supply unit. The superimposed voltage is applied to the smoothing capacitor through the rectifier diode. As a result, the output voltage of the DC power supply unit is boosted.
As shown in FIG. 1, a switching power supply circuit of this kind is made up of a battery 1, a choke coil 2, a switching element 3, a diode 4, a capacitor 5, a rectifier diode 6, a smoothing capacitor 7, and a controller 8. A load Z is connected to the smoothing capacitor 7 in parallel. In this example, the switching element 3 is realized by an n-channel MOSFET. Hereinafter, n-channel MOSFET may be referred to as nMOS.
In this switching power supply circuit, electromagnetic energy supplied from the battery 1 is stored in the choke coil 2 when the switching element 3 is in ON state. Subsequently, when the switching element 3 is turned into OFF state, counter electromotive force generated in the choke coil 2 is superimposed on the output voltage VE of the battery 1. The superimposed voltage is applied to the smoothing capacitor 7 through the rectifier diode 6. As a result, the output voltage VE of the battery 1 is boosted to be an output voltage VN, which is applied to the load Z.
The controller 8 monitors the output voltage VN. The controller 8 controls the time width of ON state of the switching element 3 in accordance with the output voltage VN to keep the output voltage VN substantially at a set value.
However, this switching power supply circuit has the following problem. That is, loss is generated when the switching element 3 is turned on or off. For example, in case of turning the switching element 3 on, immediately before the turning-on operation, the capacitor 5 is a parasitic capacitance to the switching element 3 and has been charged by substantially the same voltage as the output voltage VN. Charges stored in the capacitor 5 are released through the switching element 3 when the switching element 3 is turned on. At this time, electromagnetic energy stored in the capacitor 5 is consumed by the switching element 3. This causes power loss.
In addition, immediately before the switching element 3 is turned on, load current is flowing through the rectifier diode 6 in the forward direction. When the switching element 3 is turned on in this state, counter electromotive force is applied from the smoothing capacitor 7 to the rectifier diode 6. Thus, recovery current flows in the rectifier diode 8 and there is no restriction of the recovery current. As a result, a large amount of loss and a large amount of noise are generated. Even if the rectifier diode 6 is realized by a fast recovery diode, the recovery current can not completely be eliminated though the recovery current can be reduced.
As another switching power supply circuit than the above-described one, for example, the following switching power supply is known.
JP-A-6-311738 discloses a boost-chopper type switching power supply. As shown in FIG. 2, this switching power supply is made up of a battery 1, a choke coil 2, a main switching element 3, a diode 4, a capacitor 5, a rectifier diode 6, a smoothing capacitor 7, a controller 8A, a choke coil 9, diodes 10 and 11, an auxiliary switching element 12, and a diode 13. A load Z is connected to the smoothing capacitor 7 in parallel.
In this switching power supply, as shown in FIG. 3, the auxiliary switching element 12 is turned into ON state immediately before the main switching element 3 is turned into ON state. The auxiliary switching element 12 is turned into OFF state immediately after the main switching element 3 is turned into ON state. First, when the auxiliary switching element 12 is turned into ON state, the rising of the current flowing in the auxiliary switching element 12 is made dull by the choke coil 9. This reduces the switching loss when the auxiliary switching element 12 changes from OFF state into ON state, Next, when the current flowing in the choke coil 9 rises to be equal to the current in the choke coil 2, charges in the capacitor 5 are extracted by resonance between the choke coil 9 and the capacitor 6. When the discharge of the capacitor 5 is completed, the diode 4 is turned into ON state. Because the main switching element 3 is turned on in the period of the ON state of the diode 4, switching of the main switching element 3 is zero-voltage switching and thus the switching loss is reduced.
However, when the auxiliary switching element 12 changes from OFF state into ON state, the voltage between both ends of the auxiliary switching element 12 sharply rises due to electromagnetic energy stored in the choke coil 9. Therefore, switching loss is generated in the auxiliary switching element 12. Next, the electromagnetic energy stored in the choke coil 9 is released through the current path of the main switching element 3, the choke coil 9, and the diode 10.
As apparent from the above description, this switching supply has a problem that switching loss is generated when the auxiliary switching element 12 is turned off. If a capacitor is connected to the auxiliary switching element 12 in parallel, the rising of the voltage between both ends of the auxiliary switching element 12 can be made dull. However, because the capacitor is discharged when the auxiliary switching element 12 is turned into ON state, switching loss is generated.
Next, JP-A-7-203673 discloses a switching power supply device as shown in FIG. 4. Referring to FIG. 4, the switching power supply device is made up of a battery 1, a choke coil 2, a main switching element 3, a diode 4, a capacitor 5, a rectifier diode 6, a smoothing capacitor 7, a transformer 21, an auxiliary switching element 22, a diode 23, and capacitors 24 and 25.
FIGS. 5A to 5G show current or voltage waveforms at parts of FIG. 4. FIG. 5A shows the main switch voltage VSW1 of the main switching element 3. FIG. 5B shows the main switch current ISW1 flowing in the main switching element 3. FIG. 5C shows the auxiliary switch voltage VSW2 of the auxiliary switching element 22. FIG. 5D shows the auxiliary switch current ISW2 flowing in the auxiliary switching element 22. FIG. 5E shows the exciting current of the transformer 21. FIG. 5F shows the diode current ID1 flowing in the rectifier diode 6. FIG. 5G shows the choke current IL1 flowing in the choke coil 2.
In this switching power supply device, as shown in FIGS. 5A to 5G, when the main switching element 3 is turned into ON state at time to the voltage VE of the battery 1 is applied to the series circuit of the choke coil 2 and the primary winding n1 of the transformer 21. At this time, because the auxiliary switching element 22 is in OFF state, no current flows in the secondary winding n2 of the transformer 21. Therefore, the transformer 21 is equivalent to its exciting inductance and the same operation as that of the boost-chopper type switching power supply of FIG. 2 is performed.
When the main switching element 3 is turned into OFF state at time t1, a flyback voltage is generated on the transformer 21 and the capacitor 25 is charged through the diode 23. The voltage of the secondary winding n2 of the transformer 21 is clamped by the voltage of the capacitor 25 so that the current of the secondary winding n2 of the transformer 21 reduces linearly. At this time, the current IL1 of the choke coil 2 is supplied to a load (not shown) through the diode 6.
The current of the transformer 21 becomes zero at time t2. At this time, because the auxiliary switching element 22 is in OFF state, no current flows in the secondary winding n2 of the transformer 21. Therefore, the transformer 21 again becomes equivalent to its exciting inductance so as to start resonance with the capacitors 5 and 24. At this time, the resonance currents flow in the loop of the capacitor 5, the diode 6, and the capacitor 7, and the loop of the capacitor 24, the transformer 21, and the capacitor 25.
The auxiliary switching element 22 is turned into ON state at time t3. The voltage of the capacitor 25 is applied to the secondary winding n2 of the transformer 21 and the current of the secondary winding n2 of the transformer 21 increases linearly.
When the auxiliary switching element 22 is turned into OFF state at time t4 and no current flows in the secondary winding n2 of the transformer 21, the transformer 21 becomes equivalent to its exciting inductance so as to start resonance with the capacitor 5. At this time, the resonance current flows in the loop of the capacitor 5, the transformer 21, the diode 6, and the capacitor 7 so that the capacitor 5 is discharged. When the voltage of the capacitor 5 reaches zero or its minimum value, the main switching element 3 is turned into ON state. By this, switching of the main switching element 3 becomes zero-voltage switching or soft switching so that the switching loss is reduced considerably. At this time, because the current ID1 of the diode 6 has gotten near to zero as shown in FIG. 5F, the recovery noise is reduced that will be generated when the main switching element 3 is turned into ON state next time.
However, the switching power supply device of FIG. 4 has the following problem. That is, after the auxiliary switch current ISW2 reaches zero at time t2, the auxiliary switching element 22 is turned into ON state at time t3. Therefore, the auxiliary switching element 22 is turned into ON state when the diode 23 connected to the auxiliary switching element 22 in parallel is in OFF state. This does not bring about zero-voltage switching and therefore switching loss is generated. JP-A-7-203573 does not clearly describe the work of the diode 4 in FIG. 4.
By the way, it is thinkable to adopt the following construction in place of the battery 1 in FIG. 2 or 4. That is, in place of the battery 1, there are provided a commercial AC power supply and a rectifier circuit for rectifying the input voltage obtained from the commercial AC power supply to generate a pulsating voltage. In addition, a power factor improvement circuit is provided for controlling the input voltage to be a sine waveform substantially in phase with the input voltage. Even in this case, however, the same switching loss is generated.